1. Field of the Invention
The present invention is related to a transmission system comprising a transmitter having a speech encoder comprising means for deriving from an input signal a symbol sequence including a representation of a plurality of prediction coefficients and a representation of an excitation signal, said transmitter being coupled via a transmission medium to a receiver with a speech decoder.
The present invention is also related to a receiver, a decoder and a decoding method.
2. Description of the Related Art
A transmission system with a speech encoder and a speech decoder is known from GSM recommendation 06.10, GSM full rate speech transcoding published by European Telecommunication Standardisation Institute (ETSI) January 1992.
Such transmission systems can be used for transmission of speech signals via a transmission medium such as a radio channel, a coaxial cable or an optical fibre. Such transmission systems can also be used for recording of speech signals on a recording medium such as a magnetic tape or disc. Possible applications are automatic answering machines or dictation machines.
In modern speech transmission system, the speech signals to be transmitted are often coded using the analysis by synthesis technique. In this technique, a synthetic signal is generated by means of a synthesis filter which is excited by a plurality of excitation sequences. The synthetic speech signal is determined for a plurality of excitation sequences, and an error signal representing the error between the synthetic signal, and a target signal derived from the input signal is determined. The excitation sequence resulting in the smallest error is selected and transmitted in coded form to the receiver.
The properties of the synthesis filter are derived from characteristic features of the input signal by analysis means. In general, the analysis coefficients, often in the form of so-called prediction coefficients, are derived from the input signal. These prediction coefficients are regularly updated to cope with the changing properties of the input signal. The prediction coefficients are also transmitted to the receiver. In the receiver, the excitation sequence is recovered, and a synthetic signal is generated by applying the excitation sequence to a synthesis filter. This synthetic signal is a replica of the input signal of the transmitter.
Often the prediction coefficients are updated once per frame of samples of the speech signal, whereas the excitation signal is represented by a plurality of sub-frames comprising excitation sequences. Mostly, an integer number of sub-frames fits in one update period of the prediction coefficients. In order to improve the quality of the signal synthesised at the receiver, in known transmission system the interpolated analysis coefficients are calculated for each excitation sequence.
A second reason for using interpolation is in case one set of analysis parameters is received in error. An approximation of said erroneously received set of analysis parameters can be obtained by interpolating the level numbers of the previous set analysis parameters and the next set of analysis parameters.
Using interpolation results always in a small degradation of the speech quality when compared with a situation in which no interpolation is required because updated prediction parameters are available for each sub-frame.